Process and apparatus for the manufacture of linear high molecular weight polyesters

ABSTRACT

A process for the manufacture of high molecular weight linear polyesters which are derived from dicarboxylic acids or their ester-forming derivatives and from diols, by condensing polyester precondensates, having a relative viscosity of at least 1.05, at from 270° to 340° C. under reduced pressure, the condensation being started at from 290° to 340° C. and the temperature being lowered as the condensation progresses, the final temperature being at least 10° C. above the melting point of the particular polyester produced, and an apparatus for carrying out the process.

This is a division of application Ser. No. 932,632, filed Aug. 10, 1978.

The present invention relates to a process for the manufacture of highmolecular weight linear polyesters which are derived from dicarboxylicacids or their ester-forming derivatives and from diols, by condensingpolyester precondensates, having a relative viscosity of at least 1.05,at from 270° to 340° C. under reduced pressure.

In the manufacture of high molecular weight linear polyesters, lowmolecular weight precondensates having a low viscosity are convertedinto high molecular weight condensates at from 260° to 300° C. underreduced pressure, diols being eliminated. However, polyester melts areunstable at the high temperatures required, and this instability resultsin an increased content of carboxyl end groups. In the process disclosedin German Published Application DAS No. 1,745,541, the polyesterprecondensate is passed through a horizontal apparatus sub-divided intochambers, the melt being brought into the shape of a thin film in eachchamber. This process has the disadvantage that it requires substantialtime, eg. several hours, and that the more highly condensed melt formedin the film is continuously recycled to the bottom of the reactorcontaining material of lower molecular weight. Further, German Laid-OpenApplication DOS No. 1,959,455 discloses a process in which thecondensing melt is passed over a series of superposed zones, circulatesin each zone and in doing so comes into intermittent contact with theheating surface, and flows from zone to zone under gravity, forming afilm. The process has the disadvantage that dead spaces develop withinthe individual zones and lead to back-mixing. Further, the process hasthe disadvantage that in the case of particularly sensitive polyestersthe condensation time is still too long. The process known from FrenchPat. No. 1,545,487, in which the condensing melt is passed over aplurality of rotating inclined surfaces, still requires a condensationtime of about 30 minutes. It is noteworthy that in all the processes thetemperature is either kept constant or is increased with increasingviscosity.

It is an object of the present invention to effect the condensationreaction, in the manufacture of high molecular weight linear polyesters,in such a way as to require minimum time and as to give a very lowcontent of carboxyl end groups even in the case of sensitive polyesters.

We have found that this object is achieved by a process for themanufacture of high molecular weight linear polyesters, derived fromdicarboxylic acids or their ester-forming derivatives and from diols, bycondensing polyester precondensates, having a relative viscosity of atleast 1.05, at from 270° to 340° C. under reduced pressure, thecondensation being started at from 290° to 340° C. and the temperaturebeing lowered as the condensation progresses, the final temperaturebeing at least 10° C. above the melting point of the particularpolyester produced.

The invention further relates to an apparatus for the manufacture ofhigh molecular weight linear polyesters, characterized by a verticaltunnel (1), forming a common vapor space (2), with feed points for thepolyester precondensate (3), a discharge orifice at the lower end (4), avapor outlet at the upper end (5), and heated tubes (6) arrangedhorizontally and parallel to one another, these tubes being so arrangedbelow one another that the melt which flows downward under gravity ineach case flows over the next-lower tube, the diameter of the tubesincreasing in the downward direction.

The novel process has the advantage that the condensation takes placemore rapidly than in conventional processes. Further, it has theadvantage that even in the case of sensitive polyesters, eg.polybutylene terephthalate, very low carboxyl end group contents areachieved. The novel apparatus has the advantage that back-mixing isavoided and that there are no moving parts which may cause breakdowns.

The novel process is noteworthy in that the condensation is carried outat progressively decreasing temperature. In view of German Laid-OpenApplication DOS No. 1,920,954 and French Pat. No. 1,545,487 it was to beexpected that short residence times would only be achieved by increasingthe temperature.

The high molecular weight linear polyesters, like the polyesterprecondensates, are derived from dicarboxylic acids or theirpolyester-forming derivatives, eg. alkyl esters.

Aliphatic and/or aromatic dicarboxylic acids having a molecular weightof <390 are preferred. Particularly preferred dicarboxylic acidshave--apart from the carboxyl group--a hydrocarbon structure.Alkanedicarboxylic acids of 5 to 10 carbon atoms, andbenzenedicarboxylic acids or naphthalenedicarboxylic acids, butespecially those derived from benzene, are industrially particularlyimportant. Terephthalic acid should be singled out particularly.Examples of suitable starting materials are glutaric acid, adipic acid,sebacic acid, terephthalic acid, isophthalic acid, succinic acid,naphthalene-2,6-dicarboxylic acid, 4,4'-diphenyldicarboxylic acid,4,4'-diphenoxydicarboxylic acid and their alkyl esters, alkyl being of 1to 4 carbon atoms.

Preferred diols are aliphatic, cycloaliphatic and aromatic diols havinga molecular weight of <280. They preferably have--apart from thehydroxyl groups--a hydrocarbon structure. Alkanediols, especially thoseof 2 to 6 carbon atoms, are industrially particularly important.Examples of suitable diols are ethylene glycol, propylene glycol,1,4-butanediol, 1,6-hexanediol, 1,5-pentanediol, decamethylene glycol,neopentylgycol and 1,4-bis-hydroxymethylcyclohexane. Ethylene glycol and1,4-butanediol have become particularly important.

Preferred polyesters and their precondensates comprise at least 50 mole% of ethylene terephthalate or butylene terephthalate units.

The remainder may consist of other short-chain polyester units derivedfrom the above polyester-forming starting materials. Polyesters whichcomprise from 70 to 100 mole % of ethylene terephthalate or butyleneterephthalate units are particularly preferred.

The process of the invention is particularly important for themanufacture of polybutylene terephthalate.

The polyester precondensates are obtained in the conventional manner byreacting dicarboxylic acids or their esters with a diol in the presenceof a catalyst, eg. a titanic acid ester or antimony, manganese or zinccompound, eg. a salt of these elements with a fatty acid, at from 150°to 260° C. The resulting di(glycol) esters of carboxylic acids areprecondensed under reduced pressure at from 230° to 270° C. Theresulting precondensates have a relative viscosity of at least 1.05(measured on an 0.5 percent strength by weight solution in a mixture ofphenol and o-dichlorobenzene in the weight ratio of 3:2 at 25° C.). As arule, the polyester precondensates used as the starting materials have arelative viscosity of from 1.05 to 1.2. The manufacture of suchprecondensates is described in, for example, German Laid-OpenApplication DOS No. 2,514,116.

The polyester precondensates are condensed to give high molecular weightpolyesters at from 270° to 340° C., under reduced pressure,advantageously at from 0.1 to 2 mm Hg. Of course, the diols eliminatedduring the condensation are continuously removed from the reactionmixture.

It is an essential characteristic of the invention that the condensationis started at from 290° to 340° C. and the temperature is lowered as thecondensation progresses, the final temperature being at least 10° C.,advantageously 30° C., above the melting point of the particularpolyester produced. The starting temperature also depends on the natureof the precondensate. In the case of polyethylene terephthalate,starting temperatures of from 320° to 340° C. have proved particularlyadvantageous, whilst for polybutylene terephthalate the correspondingfigures are from 290° to 310° C. Advantageously, the high startingtemperature is lowered by from 30 to 50° C. during the condensation.This lowering may be continuous but is preferably stepwise. The finaltemperature depends essentially on the melting point of the polyesterproduced and should be sufficiently above the melting point forsolidification to be prevented and there being no interference withsubsequent processing. As a rule, we have found that temperatures ofabout 10° C. above the melting point can be used.

The process according to the invention may also be carried outadvantageously by lowering the temperature during condensationcontinuously by from 30° to 50° C., by carrying out the processadiabatically.

This procedure has the advantage that the optimum reaction temperatureis obtained substantially automatically. The novel process has thefurther advantage that it can be carried out in conventionalcondensation reactors, with an optimum temperature-time profile, if suchreactors are fitted with a heat exchanger upstream from the condensationstage of the polyester condensation.

To start with, the low-viscosity polyester precondensate melt is heated,before entering the polycondensation zone, to from 30° to 50° C. abovethe temperature which the polyester obtained after the polycondensationhas finished is to exhibit. Advantageously, the temperature of thepolyester precondensate is initially raised by the same amount as itfalls during the subsequent polycondensation. The heating of thepolyester precondensate melt is advantageously carried out rapidly, in aheat exchanger, for example a tube exchanger or plate exchanger, or insome similar suitable apparatus.

After the polyester precondensate melt which has been heated in this wayhas entered the condensation zone, the condensation starts as aconsequence of trans-esterification reactions. The heat energy requiredfor the reaction and for the evaporation of the diol liberated, and ofany by-products, eg. tetrahydrofuran in the case of the condensation ofpolyesters containing, 1,4-butanediol, is abstracted from the heatcontent of the melt. As a result, the temperature drops as thecondensation reaction progresses, so that the optimum reactiontemperature results substantially automatically. Accordingly, thesurfaces in the polycondensation zone, which come into contact with themelt, are also kept at the temperature which the polyester is to exhibitafter polycondensation and which may be up to 10° C. below the finaltemperature of the polycondensation reaction. As a result, toward theend of the polycondensation the melt is at the temperature whichcorresponds to that of the surfaces which are in contact with the meltor which may be from 5° to 10° C. above this value, depending on theconstruction of the apparatus used, due to absorption of mechanicalenergy during agitation of the melt.

The condensation is preferably carried out in a thin layer, which forthe purposes of the invention means a layer of not more than 7 mm.Advantageously, the thin layers are in contact with heated surfaces soas to ensure rapid heat transfer and rapid adaptation to the decreasingcondensation temperature. For this reason, it has proved advantageous toallow the condensing melt to flow as a thin layer, under gravity, over asubstantially vertical indirectly heated surface. In order to achieve agraduated reaction temperature from the top of the reactor to thebottom, it has proved particularly advantageous to allow the condensingmelt to flow in a thin layer, under gravity, over a plurality ofindirectly heated surfaces, a film being formed as the material flowsfrom one surface to the next. The number of surfaces required depends onthe details of the apparatus. A suitable apparatus, for example, is thatdescribed in French Pat. No. 1,545,487, provided the inclined surfacesare heated. Another suitable apparatus will be explained below.

FIG. 1 shows a cross-section through the apparatus according to theinvention for the manufacture of linear high molecular weightpolyesters. The apparatus consists of a substantially vertical tunnel(1), which may be of round or polygonal section. Advantageously, thetunnel tapers at the bottom, so as to allow the melt to accumulate. Thetunnel forms a common vapor space (2). As a result, a uniform pressureis maintained at all points of the condensation over the entirecondensation sequence. The diols eliminated are removed through thevapor outlet (5) at the upper end of the tunnel. The appropriate vaporseparators and the corresponding vacuum equipment is not shown. Anorifice (4) for discharging the polyester melt is provided at the bottomend of the tunnel, the polyester being discharged by means of gear pumpsor extruders, not shown in the drawing. Heated tubes (6) are arrangedhorizontally, and parallel to one another, in the tunnel. The tubes arearranged below one another so that the melt which flows downward undergravity in each case flows over the next-lower tube, the diameter of thetubes increasing in the downward direction of the tunnel. The polyesterprecondensate is introduced through the feed point (3) in such a waythat the melt is applied over the entire length of the tubes of theuppermost layer.

Over the length of the tunnel, the tubes are advantageously sub-dividedinto groups, the uppermost group consisting of tubes of the smallestdiameter. Each group can consist of several layers of tubes.Advantageously, the tubes in the individual groups are arranged onebelow the other. The number of tubes and of layers depends on thedimensions of the particular apparatus used. In the next group, thetubes are advantageously again arranged in individual layers, one belowthe other. Advantageously, the tube diameter increases by a factor offrom 2.0 to 4.0 from each group to the next. Further, it has provedadvantageous if the tubes of each successive group are arrangedstaggered relative to the tubes of the group above. As a result, themelt flowing down from two tubes of group a impinges on one tube ofgroup b. The melt flowing down from two tubes of group b then impingeson one tube of group c. As a result of the plurality of tubes, thedecrease in temperature in the downward direction of the tunnel caneasily be brought about, for example from group to group, or withfurther sub-division within a group.

The polyesters obtainable by the process of the invention may be usedfor the manufacture of shaped articles, eg. filaments, films orinjection-molded or extruded objects, and also for the production ofcoatings.

The Examples which follow illustrate the process of the invention.

EXAMPLES

(a) Manufacture of the polyester precondensate

1,000 g of dimethyl terephthalate and 685 g of 1,4-butanediol are heatedto 130° C. in a 2 liter round flask equipped with a stirrer, nitrogeninlet and packed column. At this temperature, 1.5 g of tetrabutylorthotitanate, to act as the trans-esterification catalyst, were added,whilst stirring. The distillation of methanol soon started. Thetemperature was raised to 220° C. in the course of 2 hours, after which330 g of methanol had distilled off and the trans-esterificationreaction had ended. The packed column was now replaced by a descendingcondenser and the temperature was raised to 250° C. over 15 minutes. Thepressure was then lowered steadily and linearly to 10 mm Hg over 40minutes, with rapid stirring. The mixture was then stirred for a further5 minutes at this pressure, after which the precondensation wasterminated by cracking the vacuum with nitrogen.

The precondensate was poured, under nitrogen, into a dish, where itsolidified rapidly. The relative viscosity of this precondensate was1.13.

(b) Condensation

The condensation was carried out in a 250 ml round flask equipped with astirrer, condenser and nitrogen inlet, and heated by a bath of Wood'smetal. For the post-condensation, 50 g of the precondensate were fusedunder nitrogen at the selected post-condensation temperature. After thematerial had melted and the temperature had reached equilibrium, thepressure in the flask was rapidly reduced to about 0.5 mm Hg. The speedof stirring was chosen to suit the particular viscosity. After apredetermined time, the post-condensation was stopped by cracking thevacuum with nitrogen.

The results are given in the Table which follows.

    ______________________________________                                                    Temperature                                                                   [°C.]                                                                            Time, min  η.sub.rel                                ______________________________________                                        Comparative examples                                                          1             255         15         1.26                                     2             270         15         1.38                                     3             280         15         1.44                                     4             290         15         1.37                                     Examples                                                                      1             295         0-2 mins                                                          290         2-4 mins                                                          285         4-6 mins                                                          280         6-8 mins                                                          275         8-11 mins                                                         270         11-14 mins 1.77                                     2             295         0-2 mins                                                          290         2-4 mins                                                          285         4-6 mins                                                          280         6-8 mins                                                          275         8-10 mins                                                         270         10-12 mins 1.67                                     ______________________________________                                    

Comparative Examples 1 to 4 to show the relative viscosity as a functionof the post-condensation temperature, after a reaction time of 15minutes. In accordance with the prior art, the temperature was keptconstant over the duration of the polycondensation. If the temperatureis raised from 255° C. to 280° C., the relative viscosity alsoincreases, as does the degree of polycondensation. On further raisingthe temperature to 290° C., the relative viscosity again decreases and ayellowish product is obtained.

In contrast, Examples 1 and 2 were carried out by the process accordingto the invention. For this purpose, the metal bath was preheated to 295°C. and after the precondensate had melted the post-condensation reactionwas started. During the latter, the temperature of the heating bath waslowered in the steps shown in the Table. Using the process of theinvention, a relative viscosity of 1.67 was reached in the course of 12minutes (Example 2).

EXAMPLE 3

A. Manufacture of a polybutylene terephthalate precondensate

20 kg of dimethyl terephthalate and 13.7 kg of 1,4-butanediol wereheated to 130° C. in a stirred kettle of 40 l capacity, equipped with astirrer, nitrogen inlet and fractionating column. At this temperature,30 g of tetrabutyl orthotitanate, to act as the trans-esterificationcatalyst, were added, whilst stirring. Thereafter, the distillation ofmethanol commenced. The temperature was raised to 220° C. over 2 hours.After this time, 6.6 kg of methanol had distilled off and thetrans-esterification reaction had ended. The temperature was raised to250° C. over 30 minutes, after which the pressure was reduced steadilyand linearly to 10 mm Hg over 45 minutes. At this stage, the relativeviscosity of the polybutylene terephthalate precondensate obtained was1.12.

B. Condensation of the polybutylene terephthalate precondensate by theprocess of the invention

The polybutylene terephthalate precondensate was forced through a plateheat exchanger and thereby heated to 285° C., and was passed into acondensation kettle of 40 liters capacity, preheated to 285° C. withDiphyl vapor. This heating-up required 10 minutes. The flow of Diphylvapor was then stopped and the pressure in the condensation kettle wasabruptly reduced to 1 mm Hg, with rapid stirring.

During the polycondensation which ensued, the temperature of the meltfell exponentially to 250° C. in the course of 33 minutes. From thistime onward, the temperature was kept at 250° C. by means of Diphylvapor. After a further 21 minutes, the vacuum was cracked and the meltwas discharged under nitrogen pressure. The relative viscosity of thepolybutylene terephthalate produced was 1.72.

COMPARATIVE EXAMPLE 5

In this experiment, the polybutylene terephthalate precondensate wasalso forced through the plate heat exchanger over 10 minutes, but thetemperature was kept at 250° C. The condensation kettle was preheated to250° C. At this temperature, after the kettle had been filled, thepressure was abruptly reduced to 1 mm Hg whilst stirring rapidly, andthe condensation was carried out under these conditions for 54 minutes.After this time, the vacuum was cracked and the melt was discharged. Therelative viscosity was only 1.49.

We claim:
 1. Apparatus for the manufacture of high molecular weightlinear polyesters, comprising:(a) means defining a substantiallyvertical tunnel, the bore of which defines a common vapor space; (b) aplurality of groups of horizontal, parallel heated tubes within the boreof said tunnel and spaced from another, wherein each group is comprisedof a plurality of layers of tubes with the tubes of each group allhaving the same diameter, wherein at least one of said groups has aplurality of tubes in each layer; (c) feed means disposed above saidtubes for introducing polyester precondensate into the bore of thetunnel; (d) means defining an orifice for the discharge of formedpolyester melt from the apparatus; and (e) a vapor outlet; wherein saidcommon vapor space is substantially free from obstructions other thanthe tubes and wherein the tubes are so arranged below one another thatthe melt flows under gravity from one tube to the next tube below it,the diameters of the tubes increasing in the downward direction of thetunnel.
 2. The apparatus of claim 1, wherein the tube diameter increasesby a factor of from 2.0 to 4.0 from each group to the next.
 3. Theapparatus of claim 1 or 2, wherein, over the length of the tunnel, thetubes of each successive group are arranged staggered relative to thepreceding group.